Rail Interface System (RIS, sometimes also referred to as Rail Accessory System, RAS) is a generic term for a system for attaching accessories to small firearms such as pistols, rifles and light machine guns.

These are used primarily in the military and by firearm enthusiasts to improve the usability of the weapon, being accessorized quickly and efficiently without requiring the operator to field-strip the weapon. Basic systems such as small rails (20mm is standard) with holes machined in them to be screwed onto the existing hand-guard of a rifle and can cost as little as US$25 to US$40. More advanced systems allow for numerous accessories to be mounted simultaneously and can cost upwards of US$200.